Double Pretest design

Description

This design can be used when random selection is not possible, yet the groups
selected are large enough to allow for random assignment from within them.

Start with non-random selection of two groups, one for treatment and the
other as a control, then randomly sub-divide each group into two sub-groups.
One sub-group from each of the treatment and control groups is given a pre-test,
then the other sub-group continues with the experiment, with the treatment group
being given the treatment and the post-test and the control group just being
given the post-test.

Example

Two school classes are taking part in an educational
experiment, one as the control and one as the treatment group. The classes are
randomly split in half and the scores from tests are shown below.

Class

Sub-group

Pre-test

Post-test

A

A1

12

A1

20

B

B2

11

B2

13

The result shows a significant improvement in class A, and that
the 'increase' of score between the two B subgroups cannot be due to priming or
learning.

Discussion

A normal non-equivalent design (with assigned, rather than random, groups)
may suffer from priming or learning, where the post-test result is improved or
otherwise affected by having taken the pre-test.

The Double Pretest has a higher internal
validity as it avoids any priming or learning effects by preventing any
person taking a test twice. It assumes that with random assignment within a
group, any variation in score between the pre-test and post-test can only be due
to random variation. This is not quite true if the pre-test and post-test is
separated by a long period in which other learning factors may occur.